1. Field of the Invention
The present invention relates to a focus detecting device of the phase difference detection type for detecting the focus state on the basis of luminance distributions of an object in two different directions.
2. Related Background Art
Heretofore, for example, a focus detecting device of the phase difference detection type disposed in a camera has been designed such that the amount of defocus of a photo-taking lens is detected from the luminance distribution of an object to be photographed in only one direction. Therefore this device has suffered from the disadvantage that focus detection is impossible for an object having no luminance distribution in that direction. In view of this, the applicant has proposed in Japanese Patent application No. 235822/1985 a device in which, for example, a longitudinal pair of focus detecting line sensors and a lateral pair of focus detecting line sensors and a secondary imaging lens having a pair of lens portions for forming secondary images on the line sensors are disposed to detect the luminance distributions of the photographing picture plane in the lateral direction and the longitudinal direction and the amounts of defocus of the photo-taking lens can be detected from the outputs of the respective sensors. Thereby, even if one amount of defocus becomes uncalculatable depending on the then appearance and pattern of the object to be photographed, it is possible to obtain the amount of defocus from the other luminance distribution and therefore, it never happens that focus detection becomes impossible, and such a device may be said to be a very effective device.
Now, in the device as described above, design the is made such that the relative spacing between the secondary images formed on the pair of line sensors used for longitudinal distance measurement is the same as the spacing between the secondary images formed on the pair of line sensors used for lateral distance measurement, but improvements in focus detection accuracy have been expected.
On the other hand, techniques for reducing the assembly error of the device are disclosed in U.S. patent application Ser. No. 7/077,998 and U.S. Pat. No. 4,555,169.